First try to read the temperature was with a Texas Instrument 16 bits ADC, ADS1118, and an Arduino.
It was a failure, because ADS1118 proves to be very unreliable when used with the schematics from it's own datasheet. I fried 3 chips. Still don't know how, but after I looked at the TI evaluation board schematic for ADS1118, I saw insane electrostatic protections for this chip. I guess it was an electrostatic failure, and I swear I never burnt a chip before in this way.
This is King ADS1118 Failure, the 3rd:
So, I looked for another 16 bits ADC, possible with a temperature sensor on it, for the cold junction compensation of the thermocouple. A K type thermocouple gives about 10 mV at 250 °C (482 °F), so a 16 bits ADC was a request in order to avoid a low voltage DC analog amplifier circuit and it's offset and temperature drift problems.
It proves out that the Freescale dev board FRDM-KL25Z was just the right thing:
- it has a 16 bits ADC
- ADC have a nice feature of hardware averaging up to 32 ADC samples, very useful to filter the voltage noises
- temperature sensor on the chip, good for thermocouple cold junction compensation
- much faster and with more memory and processing power than Arduino
- pinout of the FRDM-KL25Z dev board is compatible with Arduino shields, so the same LCD with touchscreen can be used
- native USB
With all these hardware goodies, the only drawback is that I am not so familiar with Freescale software development tools, but I really like working with something new.
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